Studies of the relationship of ventricular recovery properties of the form of the T wave of the body surface ECG are being conducted. Recovery properties are being assessed in control states, over a range of heart rates, during alteration of sympathetic tone and with a variety of cardiac drugs. Functional refractory periods are being used as the measure of recovery properties in some animals and in other animals concentric suction electrodes will be used as an approximation of action potential downstroke form. The relationship of T waveform to vulnerability to fibrillation will be analyzed by experimentally altering recovery properties, measuring T wave area, and measuring fibrillation threshold. The relations of characteristics of the excitability cycle to vulnerability to arrhythmias is also being evaluated. These studies are aimed at improving understanding of the physiologic basis of the T wave and improving the clinical utility of clinical electrocardiograms.